There are several synonyms for the term "gaff-topsail," which is a type of sail typically used on sailing vessels. Some of the common synonyms include a gaff sail, topsail yard, or simply a topsail. A gaff sail is a type of four-sided sail similar to a spanker sail, while a topsail yard is a horizontal spar that is attached to the top of a mast or other vertical structure. In general, these terms are all used to describe a specific type of sail that is used for specific purposes, such as providing additional lift or controlling the movement of a sailing vessel in different wind conditions.